1. Field of the Invention
The present invention relates to a variable delivery fuel supply device which is used for an internal combustion engine for automobiles, in particular, for a cylinder injection type gasoline engine requiring highly pressurized fuel and which is capable of controlling an amount of fuel to be supplied to fuel injection valves.
2. Discussion of Background
FIG. 7 is a systematic diagram showing the construction of a conventional variable delivery fuel supply device, and FIG. 8 is a cross-sectional view of a relief valve used for the conventional delivery fuel supply device.
The systematic diagram of FIG. 7 is disclosed, for instance, in JP-A-11-200990. In FIG. 7, reference numerals 1a through 1d designate fuel injection valves for injecting fuel into respective cylinders of an internal combustion engine, numeral 2 designates a delivery pipe for supplying fuel to the fuel injection valves, numeral 3 a fuel pump for supplying pressurized fuel to the delivery pipe 2, numeral 4 a low pressure pump for supplying fuel from a fuel tank 5 to the fuel pump 3, numeral 6 a check valve provided in a fuel passage 7, numeral 8 a low pressure regulator for controlling pressure in the fuel passage 7, numeral 9 a relief valve for relieving fuel from a fuel passage 10 through a fuel passage 11 to the fuel tank 5 when a fuel pressure in the delivery pipe 2 exceeds a predetermined value, and numeral 12 a return passage for returning fuel through the fuel pump 3 to the fuel tank 5.
The fuel pump 3 comprises a cylinder, a plunger which sucks fuel at a suction step to feed it into a pressurizing chamber by performing a reciprocating movement in the cylinder due to a driving action of a driving cam provided on a cam shaft of the internal combustion engine and supplies forcibly fuel pressurized in the pressurizing chamber at a discharge step, and an electromagnetic valve for controlling a quantity of fuel discharged from the pressurizing chamber by relieving the pressurized fuel in the pressurizing chamber to a low pressure side at a predetermined timing, whereby control to a fuel pressure in the delivery pipe 2 is conducted so as to provide a predetermined pressure value, although these structural components are not shown in the drawing.
For the relief valve 9 for relieving fuel from the delivery pipe 2 to the fuel tank 5 when a fuel pressure exceeds a predetermined value, the relief valve as shown in FIG. 8 is generally used. The relief valve 9 in FIG. 8 comprises a housing 15 having a connecting port 13 at a high pressure side and a connecting port 14 at a low pressure side, a valve 16, a valve seat 17 and a spring 18 which usually urges the valve 16 toward the valve seat 17 to make a closing state. In a normal condition of use, a fuel pressure in the delivery pipe 2 is controlled by the electromagnetic valve. Since the relief valve 9 relieves fuel in the delivery pipe 2 only when a fuel pressure in the delivery pipe 2 is in an abnormal state, a check valve of small relief capacity is used.
In the conventional variable fuel supply device having the construction described above, when a starting operation is conducted to the internal combustion engine, the low pressure pump 4 is actuated so that fuel is supplied from the fuel tank 5 to the fuel pump 3, and the fuel pump 3 is driven by the internal combustion engine, whereby the fuel pump 3 sucks fuel into the pressurizing chamber at a suction step of the plunger and supplies forcibly pressurized fuel through the fuel passage 10 to the delivery pipe 2 at a discharge step. In a normal operating condition, a fuel pressure in the delivery pipe 2 is controlled by the electromagnetic valve located in the fuel pump 3 to have a predetermined pressure value. However, since there is a limit in the response characteristics of the electromagnetic valve, a fuel pressure in the delivery pipe 2 may increase abnormally depending on operational conditions of the internal combustion engine. In such case, the relief valve 9 is actuated to relieve fuel so that breakage of a high-pressure fuel passage such as the delivery pipe 2, the fuel passage 10 or the like can be prevented.
Although the above-mentioned conventional variable fuel supply device achieved a stable control of fuel pressure in a normal operating condition, there was a case that for example, when the electromagnetic valve became faulty such as inoperability of fuel pressure control, an amount of fuel which exceeds the limit of the capacity of the relief valve 9 might be supplied from the fuel pump 3 to the delivery pipe 2. This caused an abnormal increase of fuel pressure in the high-pressure fuel passage such as the delivery pipe 2, whereby structural elements constituting the high-pressure fuel passage might be broken. Further, even when the electromagnetic valve correctly operated, there was difficulty in estimating practically a fuel pressure in the delivery pipe 2 or the fuel pump in a course of raising a fuel pressure in the delivery pipe 2 to a predetermined pressure value at the time, for instance, of the starting of the internal combustion engine. In such case, when the electromagnetic valve was actuated, a fuel pressure in the delivery pipe 2 sometimes became unstable.
The electromagnetic valve performs normally relieving operations at each step of the operations of fuel pump. Since it operates continuously during the operations of the internal combustion engine, the frequency of operation is extremely high. Therefore, occurrence of a defect such as inoperability of fuel pressure control due to, for instance, the wearing of a constitutional element is unavoidable. When such defect occurs, an amount of fuel which is beyond the limit of the performance of the relief valve 9 is supplied in a high-rotational region of the fuel pump, and the fuel pressure abnormally rises, whereby the high-pressure passage such as the delivery pipe 2 may be broken. In order to eliminate such defect and improve the durability of the electromagnetic valve, it is an effective way to decrease the operational frequency of the electromagnetic valve.